1. Field of the Invention
The present invention relates to a spline telescopic shaft, a method for manufacturing the spline telescopic shaft, and a vehicle steering apparatus.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 2005-153677 discusses a telescopic shaft for vehicle steering. The telescopic shaft for vehicle steering has a surface hardened layer provided on its tooth surface via shot peening, and a large number of minute recesses are formed on the surface hardened layer. The recesses function as a grease reservoir.
In this type of spline telescopic shaft, resin coatings may be respectively formed on tooth surfaces of an inner shaft and an outer shaft. Frictional resistance in engagement between teeth of the inner shaft and teeth of the outer shaft is reduced by forming the resin coatings. Thus, friction between both the teeth is suppressed. Backlash between both the shafts in the early stages of use of the telescopic shaft is reduced. A sliding load between both the shafts can be reduced. Therefore, work for assembling the telescopic shaft into a vehicle is simplified. Moreover, durability is improved. Further, stick slip vibration between both the shafts is suppressed through reduction of a change in the sliding load between both the shafts. This improves quietness.